I remember my first yo-yo: a blue Duncan Imperial. I was 7 years old and had saved up enough of my allowance to buy it. The drive to the store felt like an eternity. When I finally opened the package, the bright, shiny yo-yo smelled of plastic and felt as smooth as ice—it was perfect. Back at home, I spent hours in the driveway playing with my new toy.

What is it about light that makes it so appealing as an element of play? One of my recent blogs focused upon glow-in-the-dark toys that use phosphors and a form of light emission known as photoluminescence to provide a familiar greenish glow.